Light-emitting devices can be used, for example, in displays (e.g., flat-panel displays), screens (e.g., computer screens), and other items that require illumination. Accordingly, the brightness of the light-emitting device is an important feature of the device. Also, low operating voltages and high efficiencies can improve the viability of producing emissive devices. In many applications, long device lifetime is desirable.
Light-emitting devices can release photons in response to excitation of an active component of the device. Emission can be stimulated by applying a voltage across the active component (e.g., an electroluminescent component) of the device. Some devices can be operated with a time-varying voltage, i.e., an alternating current; such devices can be referred to as alternating current thin-film electroluminescent (AC-TFEL) devices. The electroluminescent component can be a polymer, such as a conjugated organic polymer or a polymer containing electroluminescent moieties or layers of organic molecules. Typically, the emission can occur by radiative recombination of an excited charge between layers of a device. The emitted light has an emission profile that includes a maximum emission wavelength, and an emission intensity, measured in luminance (candelas/square meter (cd/m2) or power flux (W/m2)). The emission profile, and other physical characteristics of the device, can be altered by the electronic structure (e.g., energy gaps) of the material. For example, the brightness, range of color, efficiency, operating voltage, and operating half-lives of light-emitting devices can vary based on the structure of the device.
Today, many flat panel displays use liquid crystal displays (LCDs) which rely on a backlight and subtractive processing of light to produce a range of colors. Although the power requirements of LCDs are reasonably low (due to efficient backlighting), the subtractive process inherently limits light efficiency. The subtractive process also limits the darkest black that can be produced, and thus the contrast ratio. In contrast, organic light emitting diodes (OLEDs) can provide additive rather than subtractive displays. Blacks are blacker, and less power can be required as less light is lost due to absorption.
However, most organic light emitting diodes currently employed in displays (e.g., portable displays, such as those found in mobile phones and digital music players) can be sensitive to air and moisture. To address this issue, devices are typically embedded in polymer packaging to be employed in portable device displays.